Crack closure and fatigue crack growth near threshold of a metastable austenitic stainless steel

• The Donald’s effect establish a good correlation between driving force and FCGR.
• The strange values of ΔKth found in MASS can be explained by the crack closure.
• For this material the crack closure is induced by roughness.
• To explain the load ratio effects in MASS is vital to include Kmax and crack closure.
• The influence of Kmax and ΔK on the FCG vary with the change in the FCGR.

In this paper R-ratio effects on fatigue crack growth near threshold region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and Kmax = 23 MPa√m using a two-parameters approach (ΔK, Kmax and α in Kujawski’s model) and crack closure model (using Elber’s Kop and in Donald’s ACRn2 approaches). The Kop and ACRn2 were experimentally measured on a single edge tension specimens. The Kop measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris region, they fail to correlate them in the threshold region. However, this correlation can be improved in the threshold region when a different α value from the Paris region is used. The authors indicated that two different mechanisms operate; one in the Paris region and another in the near threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔKeff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify Kmax in the two-parameter approach.